1. Field of the Disclosure
Aspects of the present disclosure relate to ultrasonic transducers, and, more particularly, to a method of forming a piezoelectric micromachined ultrasonic transducer defining an air-backed cavity, and an associated apparatus.
2. Description of Related Art
Some micromachined ultrasonic transducers (MUTs) may be configured, for example, as a piezoelectric micromachined ultrasonic transducer (pMUT) disclosed in U.S. Pat. No. 7,449,821 assigned to Research Triangle Institute, also the assignee of the present disclosure, which is also incorporated herein in its entirety by reference.
The formation of a pMUT device, such as the pMUT device defining an air-backed cavity as disclosed in U.S. Pat. No. 7,449,821, may involve the formation of an electrically-conductive connection between the first electrode (i.e., the bottom electrode) of the transducer device, wherein the first electrode is disposed on the front side of the substrate opposite to the air-backed cavity of the pMUT device, and the conformal metal layer(s) applied to the air-backed cavity for providing subsequent connectivity, for example, to an integrated circuit (“IC”) or a flex cable. In this regard, some prior art methods involve, for example, deposition of a conformal metal layer in the air-backed cavity of the pMUT in direct contact with the first/bottom electrode (see, e.g., FIG. 7A of U.S. Pat. No. 7,449,821). In another example, the conformal metal layer is deposited in a via formed in a dielectric film to expose the first/bottom electrode (see, e.g., FIG. 7B of U.S. Pat. No. 7,449,821). In yet another example, involving a silicon-on-insulator (SOI) substrate, the conformal metal layer is deposited in a via extending to immediately adjacent the transducer device (see, e.g., FIGS. 14 and 15 of U.S. Pat. No. 7,449,821). However, such approaches may be limiting for high frequency transducer arrays having smaller dimensions.
High frequency transducers may be used for high resolution ultrasound imaging, for example, imaging with a resolution of approximately 100 μm, by operating the transducer within a frequency range of 20 MHz to 50 MHz. Transducers operating at standard imaging frequencies of, for instance, less than 10 MHz, typically have resolution of greater than 1 mm. The operating frequency for pMUT transducers may be inversely proportional to the width of the transducer element. As such, relatively smaller pMUT transducers can generally be operated at higher frequencies and, therefore, may provide a relatively better resolution. Further, in instances of steerable phased arrays, the transducer element pitch must be less than one wavelength in order to prevent grating lobe artifacts in the resulting images produced from the transducer signal. Generally, the ultrasound wavelength in bodily tissue is about 75 μm for a frequency of about 20 MHz and about 30 μm for a frequency of about 50 MHz. Therefore, it may be desirable for some transducer devices to include a transducer element having a lateral dimension (i.e., width) on the order of about 40 μm for about 20 MHz operation, or on the order of about 20 μm for operation at ultrasound frequencies of about 40-50 MHz. In such instances, however, the disclosed prior art configurations may result in an aspect ratio between the thickness of the substrate (i.e., about 400 μm) and the width of the via extending through the substrate to the transducer element of between about 10:1 and about 20:1. Such a configuration may not be desirable for forming though-wafer interconnects in line with the transducer elements (i.e., through the pMUT air-backed cavity).
Another aspect of some prior art methods is that the element forming the electrically-conductive connection between the first electrode and the conformal metal layer may be formed about one of the lateral edges of the pMUT device (see, for example, FIG. 15 of U.S. Pat. No. 7,449,821). In such instances, mechanical flexure of the actuated pMUT device may initiate or accelerate fatigue of the engagement between the electrically-conductive connection element and conformal conductive layer within the air-backed cavity of the pMUT device (otherwise referred to herein as the “second via”), for instance, due to stress concentrations about the sidewall/endwall edge of the second via. Such fatigue could result in cracking or delamination of the metal layer and failure of the electrically-conductive engagement therebetween and would thus create an open circuit condition between the first/bottom electrode of the pMUT device and the IC, flex cable or redistribution substrate engaged therewith, or may adversely affect the acoustic signals generated by the pMUT device.
Further, having a different material (i.e., a metal) disposed about a lateral edge of the membrane of the pMUT device for providing the electrical connection could change the boundary condition for membrane flexing and thus affect the frequency and/or vibrational mode (i.e., the fundamental or harmonic mode) of the pMUT device. Such a configuration may particularly adversely affect smaller membranes required for high frequency operation. Applying a conformal metal layer deposition, for example, of about 2 μm in thickness to a membrane of about 20 μm in width and about 8 μm in thickness may reduce the effective free membrane width by about 20% and increase the membrane thickness by about 25%. Such a configuration, as a result, may increase the resonance frequency, but could also undesirably reduce the acoustic output due to increased stiffness of the vibrating membrane resulting from the reduced free width and increased thickness thereof.
Thus, there exists a need in the ultrasonic transducer art, particularly with respect to a piezoelectric micromachined ultrasound transducer (“pMUT”) having an air-backed cavity, for improved methods of forming an electrically-conductive connection between the first electrode (i.e., the bottom electrode) of the transducer device and the conductive member extending from the back side of the substrate to the first electrode so as to provide subsequent connectivity, for example, to an integrated circuit (“IC”) or a flex cable.